The present invention relates generally to the field of marking devices, and more particularly to a device capable of applying a marking material to a substrate by introducing the marking material into a high-velocity propellant stream.
Ink jet is currently a common printing technology. There are a variety of types of ink jet printing, including thermal ink jet (TIJ), piezo-electric ink jet, etc. In general, liquid ink droplets are ejected from an orifice located at a one terminus of a channel. In a TIJ printer, for example, a droplet is ejected by the explosive formation of a vapor bubble within an ink-bearing channel. The vapor bubble is formed by means of a heater, in the form of a resistor, located on one surface of the channel.
We have identified several disadvantages with TIJ (and other ink jet) systems known in the art. For a 300 spot-per-inch (spi) TIJ system, the exit orifice from which an ink droplet is ejected is typically on the order of about 64 xcexcm in width, with a channel-to-channel spacing (pitch) of about 84 xcexcm, and for a 600 dpi system width is about 35 xcexcm and pitch of about 42 xcexcm. A limit on the size of the exit orifice is imposed by the viscosity of the fluid ink used by these systems. It is possible to lower the viscosity of the ink by diluting it in increasing amounts of liquid (e.g., water) with an aim to reducing the exit orifice width. However, the increased liquid content of the ink results in increased wicking, paper wrinkle, and slower drying time of the ejected ink droplet, which negatively affects resolution, image quality (e.g., minimum spot size, inter-color mixing, spot shape), etc. The effect of this orifice width limitation is to limit resolution of TIJ printing, for example to well below 900 spi, because spot size is a function of the width of the exit orifice, and resolution is a function of spot size.
Another disadvantage of known ink jet technologies is the difficulty of producing greyscale printing. That is, it is very difficult for an ink jet system to produce varying size spots on a printed substrate. If one lowers the propulsive force (heat in a TIJ system) so as to eject less ink in an attempt to produce a smaller dot, or likewise increases the propulsive force to eject more ink and thereby to produce a larger dot, the trajectory of the ejected droplet is affected. This in turn renders precise dot placement difficult or impossible, and not only makes monochrome greyscale printing problematic, it makes multiple color greyscale ink jet printing impracticable. In addition, preferred greyscale printing is obtained not by varying the dot size, as is the case for TIJ, but by varying the dot density while keeping a constant dot size.
Still another disadvantage of common ink jet systems is rate of marking obtained. Approximately 80% of the time required to print a spot is taken by waiting for the ink jet channel to refill with ink by capillary action. To a certain degree, a more dilute ink flows faster, but raises the problem of wicking, substrate wrinkle, drying time, etc. discussed above.
One problem common to ejection printing systems is that the channels may become clogged. Systems such as TIJ which employ aqueous ink colorants are often sensitive to this problem, and routinely employ non-printing cycles for channel cleaning during operation. This is required since ink typically sits in an ejector waiting to be ejected during operation, and while sitting may begin to dry and lead to clogging.
Other technologies which may be relevant as background to the present invention include electrostatic grids, electrostatic ejection (so-called tone jet), acoustic ink printing, and certain aerosol and atomizing systems such as dye sublimation.
The present invention is a novel method for applying a marking material to a substrate, directly or indirectly, which overcomes the disadvantages referred to above, as well as others discussed further herein. In particular, the present invention is a method of the type including a propellant which travels through a channel, and a marking material which is controllably (i.e., modifiable in use) introduced, or metered, into the channel such that energy from the propellant propels the marking material to the substrate. The propellant is usually a dry gas which may continuously flow through the channel while the marking apparatus is in an operative configuration (i.e., in a power-on or similar state ready to mark). The system is referred to as xe2x80x9cballistic aerosol markingxe2x80x9d in the sense that marking is achieved by in essence launching a non-colloidal, solid or semi-solid particulate, or alternatively a liquid, marking material at a substrate. The shape of the channel may result in a collimated (or focused) flight of the propellant and marking material onto the substrate.
In our method, the propellant may be introduced at a propellant port into the channel to form a propellant stream. A marking material may then be introduced into the propellant stream from one or more marking material inlet ports. The propellant may enter the channel at a high velocity. Alternatively, the propellant may be introduced into the channel at a high pressure, and the channel may include a constriction (e.g., de Laval or similar converging/diverging type nozzle) for converting the high pressure of the propellant to high velocity. In such a case, the propellant is introduced at a port located at a proximal end of the channel (the converging region), and the marking material ports are provided near the distal end of the channel (at or further down-stream of a region defined as the diverging region), allowing for introduction of marking material into the propellant stream.
In the case where multiple ports are provided, each port may provide for a different color (e.g., cyan, magenta, yellow, and black), pre-marking treatment material (such as a marking material adherent), post-marking treatment material (such as a substrate surface finish material, e.g., matte or gloss coating, etc.), marking material not otherwise visible to the unaided eye (e.g., magnetic particle-bearing material, ultra violet-fluorescent material, etc.) or other marking material to be applied to the substrate. The marking material is imparted with kinetic energy from the propellant stream, and ejected from the channel at an exit orifice located at the distal end of the channel in a direction toward a substrate.
One or more such channels may be provided in a structure which, in one embodiment, is referred to herein as a print head. The width of the exit (or ejection) orifice of a channel is generally on the order of 250 xcexcm or smaller, preferably in the range of 100 xcexcm or smaller. Where more than one channel is provided, the pitch, or spacing from edge to edge (or center to center) between adjacent channels may also be on the order of 250 xcexcm or smaller, preferably in the range of 100 xcexcm or smaller. Alternatively, the channels may be staggered, allowing reduced edge-to-edge spacing. The exit orifice and/or some or all of each channel may have a circular, semicircular, oval, square, rectangular, triangular or other cross sectional shape when viewed along the direction of flow of the propellant stream (the channel""s longitudinal axis).
The material to be applied to the substrate may be transported to a port by one or more of a wide variety of ways, including simple gravity feed, hydrodynamic, electrostatic, or ultrasonic transport, etc. The material may be metered out of the port into the propellant stream also by one of a wide variety of ways, including control of the transport mechanism, or a separate system such as pressure balancing, electrostatics, acoustic energy, ink jet, etc.
The material to be applied to the substrate may be a solid or semi-solid particulate material such as a toner or variety of toners in different colors, a suspension of such a marking material in a carrier, a suspension of such a marking material in a carrier with a charge director, a phase change material, etc. One preferred embodiment employs a marking material which is particulate, solid or semi-solid, and dry or suspended in a liquid carrier. Such a marking material is referred to herein as a particulate marking material. This is to be distinguished from a liquid marking material, dissolved marking material, atomized marking material, or similar non-particulate material, which is generally referred to herein as a liquid marking material. However, the present invention is able to utilize such a liquid marking material in certain applications, as otherwise described herein.
In addition, the ability to use a wide variety of marking materials (e.g., not limited to aqueous marking material) allows the present invention to mark on a wide variety of substrates. For example, the present invention allows direct marking on non-porous substrates such as polymers, plastics, metals, glass, treated and finished surfaces, etc. The reduction in wicking and elimination of drying time also provides improved printing to porous substrates such as paper, textiles, ceramics, etc. In addition, the present invention may be configured for indirect marking, for example marking to an intermediate transfer roller or belt, marking to a viscous binder film and nip transfer system, etc.
The material to be deposited on a substrate may be subjected to post ejection modification, for example fusing or drying, overcoat, curing, etc. In the case of fusing, the kinetic energy of the material to be deposited may itself be sufficient to effectively either soften or melt (generically referred to herein as xe2x80x9cmeltxe2x80x9d) the marking material upon impact with the substrate and fuse it to the substrate. The substrate may be heated to enhance this process. Pressure rollers may be used to cold-fuse the marking material to the substrate. In-flight phase change (solid-liquid-solid) may alternatively be employed. A heated wire in the particle path is one way to accomplish the initial phase change. Alternatively, propellant temperature may accomplish this result. In one embodiment, a laser may be employed to heat and melt the particulate material in-flight to accomplish the initial phase change. The melting and fusing may also be electrostatically assisted (i.e., retaining the particulate material in a desired position to allow ample time for melting and fusing into a final desired position). The type of particulate may also dictate the post ejection modification. For example, UV curable materials may be cured by application of UV radiation, either in flight or when located on the material-bearing substrate.
Since propellant may continuously flow through a channel, channel clogging from the build-up of material is reduced or eliminated (the propellant effectively continuously cleans the channel). In addition, a closure may be provided which isolates the channels from the environment when the system is not in use. Alternatively, the print head and substrate support (e.g., platen) may be brought into physical contact to effect a closure of the channel. Initial and terminal cleaning cycles may be designed into operation of the printing system to optimize the cleaning of the channel(s). Waste material cleaned from the system may be deposited in a cleaning station. However, it is also possible to engage the closure against an orifice to redirect the propellant stream through the port and into the reservoir to thereby flush out the port.
Thus, the present invention and its various embodiments provide numerous advantages discussed above, as well as additional advantages which will be described in further detail below.